The class I and class II cell membrane glycoproteins encoded by the major histocompatibility complex (MHC) form highly polymorphic binding sites which present peptide antigens to T cells. Class I MHC molecules present antigens made within the cell while class II molecules present peptides derived from exogenous antigens taken into the cell by endocytosis. Newly synthesized class I molecules bind peptides in a pre-golgi compartment and in some unknown way antigens made in the cytosol have to translocate a membrane to gain access to the class I molecule. The MHC also encodes heat shock proteins (hsp) and ATP dependent transporter molecules related to P glycoproteins which may be involved in peptide translocation and binding to class I and class II. The class II molecules associate with a non- polymorphic molecule, invariant chain, in the endoplasmic reticulum which dissociates in a post-golgi location. Invariant chain may protect the binding site of class II or be involved in trafficking. In addition to class I and class II there are a large number of related MHC encoded molecules of unknown function. It is possible that there are other antigen processing and MHC presentation systems in addition to the class I and class II pathways that remain to be discovered. This meeting focuses entirely on the MHC. We plan to cover every aspect of its interactions without going into the T cell receptor phenomenology in much depth...except to ask what gamma/delta T cells recognize. Sessions in the meeting will cover peptide binding, superantigens, accessory molecules which bind MHC, NK cell dependence, bone-marrow resistance (Hh) and "non-classical" antigens encoded by the MHC. A discussion of all of the known interactions of the MHC glycoproteins will facilitate an understanding of the entire system.